Observations of the redshift distribution of galaxies in narrow
(~ 40 arcmin) pencil-beam surveys to
z 0.3
(Broadhurst et al. 1990;
hereafter BEKS) reveal a highly clumped and apparently
periodic distribution of galaxies. The distribution features peaks of
galaxy counts with an apparently regular separation of 128 Mpc, with
few galaxies between the peaks. What is the origin of this clumpy,
periodic distribution of galaxies? What does it imply for the nature
of the large-scale structure and the properties discussed above?
Bahcall (1991)
investigated these questions observationally, by
comparing the specific galaxy distribution with the distribution of
known superclusters.

Bahcall showed that the observed galaxy clumps originate from the
tails of large superclusters
(Section 9.1). When the narrow-beams
intersect these superclusters, which have a mean separation of
~ 100 Mpc, the BEKS galaxy distribution is reproduced.

The redshift distribution of the superclusters is essentially
identical to the galaxy redshift distribution, i.e., it reproduces the
observed peaks in the BEKS survey, for
z 0.1. This
indicates that the galaxy clumps observed in the pencil-beam survey
originate from these superclusters as the beam crosses the
superclusters' surface. The main superclusters that contribute to the
clumps were identified. For example, the first northern clump
originates from the Coma-Hercules supercluster (= the Great-Wall); the
second northern clump is mostly due to the large Corona Borealis
supercluster.

The narrow-beam survey of BEKS is directed toward the north and south
galactic poles. Some of the Bahcall-Soneira superclusters coincident
with the BEKS peaks are located at projected distances of up to ~ 50-100
Mpc from the poles. This suggests that the high-density
supercluster regions are embedded in still larger halo surfaces, ~ 100
Mpc in size, and that these large structures surround large
underdense regions. The observed number of clumps and their mean
separation are consistent with the number density of superclusters and
their average extent (Section 9.1).

The narrow widths of the BEKS peaks are consistent with, and imply,
flat superclusters. From simulations of superclusters and
pencil-beams,
Bahcall, Miller, and
Udomprasert (1996)
find that the
observed peak-widths distribution is consistent with that expected of
randomly placed superclusters with
15 Mpc width and
~ 150 Mpc extent.

The Bahcall-Soneira superclusters may exhibit weak positive
correlations on scales ~ 100-150 Mpc
(Bahcall and Burgett
1986).
This implies that the superclusters, and thus their related galaxy
clumps, are not randomly distributed but are located in some weakly
correlated network of superclusters and voids, with typical mean
separation of ~ 100 Mpc. This picture is consistent with
statistical analyses of the BEKS distribution as well as with the
observational data of large-scale structure. The apparent periodicity
in the galaxy distribution suggested by BEKS is expected to be greatly
reduced when pencil-beams in various directions are combined; the
scale reflects the typical mean separation between large
superclusters, ~ 100-150h-1 Mpc, but with large variations
at different locations.